Flexible PU foams are produced from formulations typically containing an isocyanate reactant, one or more blowing agents including water, a foam stabilizer, catalysts, and polyether polyols or polyester polyols or a mixture of these polyols.
Those skilled in the art have known for some time that the density of a foam can be controlled by carefully regulating the proportion of water in the formulation. However, the reaction of water with isocyanate compounds yields not only CO.sub.2 gas (which causes expansion of the foam), but also a urea-type product which increases the firmness of the foam. The use of water as a blowing agent thus does not permit the variation of firmness independently of density. Also, the water/isocyanate reaction generates a significant amount of heat. Thus, the maximum amount of water which can be used is limited by safety considerations and by the foam properties desired.
For years this problem has been circumvented by including in the formulations certain low-boiling, nonreactive liquids, e.g., methylene chloride or chlorofluorocarbons. Such auxiliary blowing agents permit the lowering of density without an increase in firmness, resulting in soft, stable, resilient foams. However, the use of these agents is now seen as undesirable, due to concern about the health effects of exposure to methylene chloride vapors and to concern about adverse environmental effects, specifically, depletion of the atmospheric ozone layer which may result from the release of chlorofluorocarbons into the atmosphere.
Consistent with well-established principles of polymer chemistry, the firmness of a foam (or of any other polymer) can be reduced during production by decreasing the average "crosslink density." For example, softer PU foams are obtained by increasing the average molecular weight of the polyether triols or polyester triols typically used in foam formulations, thereby reducing the number of reactive sites. Similarly, in formulations containing crude TDI (tolylene diisocyanate) or polymeric MDI (methylene diparaphenylene isocyanate), the functionality of the isocyanate compound can be decreased to achieve some softening. In formulations which instead contain distilled TDI, softness can be somewhat enhanced by limiting the excess amount of TDI present (i.e., by lowering the TDI index) or by increasing the ratio of 2,4-isomer to 2,6-isomer.
All of the above techniques are well-known in the art. All, however, are limited in the degree of softness which can be attained without adversely affecting density and other physical properties, or without creating manufacturing problems.
Another method for decreasing average crosslink density is the blending of polyether diols with the polyether triols generally used in foam formulations. As with the techniques already discussed, this offers only a limited degree of softening. Also, this blending method requires a manufacturing process which is capable of making extensive changes in the delivery rates of two polyols each time a foam having a different grade of firmness is to be produced. Since it is common industrial practice to switch from production of one foam grade to another "on the fly," i.e., without interrupting the flow of materials to the mixing head, this blending technique is commercially unattractive.